There is a continuing need for higher performance digital data communications systems. Perhaps nowhere is this need more evident than on the worldwide packet data communications network now commonly referred to as the “Internet.” On the Internet, the “richness” of content is constantly increasing, requiring an ever-increasing amount of bandwidth to provide Internet content to users. As a result of this increased demand for bandwidth, significant efforts have been made to develop new types of high-speed digital data communications systems. For example, optical fiber based networks are being built in many large metropolitan areas and undersea to connect continents. As another example, new wireless protocols are being developed to provide Internet content to many different types of small, portable devices.
One of the significant drawbacks of deploying many of these new types of high-speed digital data communications systems is the high cost and amount of time required to develop and build out the new infrastructure required by the systems. Because of these high costs, many new high-speed digital data communications systems are initially deployed only in densely populated areas, where the cost of building out the new infrastructure can be quickly recovered. Less populated areas must often wait to receive the new communications systems and some rural areas never receive the new systems where it is not cost effective to build the infrastructure.
Significant efforts are being made to utilize conventional twisted pair telephone lines to provide high-speed digital data transmission. The use of twisted pair telephone lines provides two important benefits. First, a significant amount of twisted pair telephone line infrastructure already exists in many countries. Thus, using conventional twisted pair telephone lines avoids the cost of building expensive new infrastructure. Second, conventional twisted pair telephone lines extend into customers' homes and businesses, avoiding the so-called “last mile” problem. As a result of recent development efforts in this area, several new communications protocols, such as Asymmetric Digital Subscriber Line (ADSL), G.Lite and Very high speed Digital Subscriber Line (VDSL), have been developed for providing high-speed digital transmission over conventional twisted pair telephone lines.
Despite the advantages to using conventional twisted pair telephone lines to provide high-speed digital communications, there are some problems with this approach. First, conventional twisted pair telephone lines cause signal attenuation per unit length that increases rapidly with frequency. A moderate length twisted pair line, for example around fifteen thousand feet, may cause only a few decibels (dB) of attenuation in the voice band, for which the line was originally designed, but many tens of dB of attenuation at higher transmission frequencies, for example around 1.1 MHz for ADSL. This results in a transfer function with a wide dynamic range, making channel equalization more difficult. The transfer function is further complicated by bridge taps and impedance mismatches between line sections that cause reflections and echoes at the receiver. Furthermore, filtering performed at the transmitter and receiver also increases the complexity of the transfer function.
The standards for ADSL and G.Lite specify Discrete Multitone (DMT) modulation. DMT is also under consideration for use in VDSL systems. DMT modulation generally involves transmitting digital data on a number of carriers simultaneously. Modulation and demodulation are performed using a Fast Fourier Transform (FFT). A cyclic prefix is introduced to ensure separation between successive DMT symbols and eliminate inter-symbol interference (ISI). In practice, the cyclic prefix is necessarily quite short, generally much shorter than the impulse response of the communications channel. This often results in significant ISI being present in the received data. Large amounts of ISI cause a large reduction in the available communications bandwidth. This is especially true for long twisted pair telephone lines likely to be encountered in ADSL and VDSL communications systems. The effect of this ISI is to reduce the Signal-to-Noise Ratio (SNR) in each bin of the FFT demodulator employed in a DMT system.
Standard equalizers used in digital communication systems, such as adaptive Least Means Squares (LMS) and Recursive Least Squares (RLS) equalizers, are generally inappropriate for DMT systems since they do not adequately shorten the communications channel impulse response and thereby fail to sufficiently eliminate ISI. Some attempts have been made to shorten the overall channel plus equalizer impulse response to be less than the cyclic prefix length. See for example, A Multicarrier Primer, by J. M. Cioffi; and Impulse Response Shortening for Discrete Multitone Transceivers, by P. Melsa, R. Younce and C. Rohrs, IEEE Transactions on Communications, Vol. 44, No. 12, Dec. 1996.
In addition to the equalization problem, twisted pair lines suffer from various forms of interference. Up to fifty twisted pairs are conventionally grouped together in binders. As a result, a signal on one pair can cause interference on other pairs in the same binder. This interference is called crosstalk and causes a reduction in the SNR at the receiver. Current approaches to mitigate crosstalk require access to the signal transmitted on the interfering line. This makes current approaches useful only in a central office environment, where the signals on all pairs in a binder are available. Thus, none of the existing crosstalk mitigation approaches are suitable when only the received signal is available. The foregoing problems are exacerbated in dynamic communications environments. For example, equalizer performance may be further degraded by changes on the transmission medium, e.g., the transmission line, attributable to temperature changes or changes in the interference sources.
Based on the foregoing, there is a need for an approach for processing data received from a communications channel that does not suffer from the limitations of conventional approaches.